Toast peppercorns, cumin, and allspice together, then cool slightly. Transfer to grinder along with annato seeds, and grind to a powder. Transfer to a small bowl.

Using the side of a large heavy chef’s knife, mince garlic and mash to a paste with remaining 1-1/2 teaspoons salt. Add to ground spices, along with oregano and remaining 6 Tbsp juice. Stir to make a paste.

Toss pork with paste to coat well. Add onion and toss to combine.

Holding both ends of a banana leaf, drag leaf over a burner on moderately high heat slowly until it changes color slightly and becomes shinier, then turn over and toast other side. Toast remaining banana leaves in same manner.